In recent years, as image-recording materials, materials for forming color images have been predominant and, specifically, recording materials for an ink jet system, recording materials for a thermal transfer system, recording materials for an electrophotographic system, transfer type silver halide light-sensitive materials, printing inks, and recording pens have found widespread use. Also, in photographing devices such as CCDs for photographing equipment, and in LCDs and PDPs for display, color filters are used for recording or reproducing a color image. In these color image recording materials and color filters, colorants (dyes or pigments) of three primary colors of a so-called additive color mixing process or subtractive color mixing process have been used in order to display or record full-color images. In actuality, however, there is no fast colorant having the absorption characteristics capable of realizing a preferred color reproduction region and resisting various use conditions and environmental conditions. Thus, the improvement thereof has strongly been desired.
In particular, use of recording materials has extended from domestic use to industrial use and, as a result, they are required to have performance at a higher level (regarding hue, tinctorial strength, and image fastness to light, gas, heat, moisture, and chemicals).
With respect to coloring materials to be used (for example, an ink for inkjet recording), dye inks are required to be changed from water-soluble inks to oil-soluble inks and, in the case where a much higher level of performance is required (from indoor use to outdoor use), the inks are required to be changed from dye inks to pigment inks.
With respect to outdoor uses, not only preserving properties of each single color but well-balanced preserving properties between respective colors and well-balanced preserving properties with each color in low density regions to high density regions are required to be provided at higher levels than in indoor uses.
Difference between dyes and pigments in using manner is that, while dyes are used in a state of being dissolved (a state of molecular dispersion) in a medium such as fibers or solvents, pigments are used in a state of solid particles (molecular aggregate) finely dispersed in a medium without being dissolved.
The dyes or pigments to be used for the above-mentioned uses are required to have in common the following properties. That is, they are required to have absorption characteristics favorable in view of color reproduction and have good fastness under the conditions of the environment wherein they are used, for example, fastness against light, heat, and an oxidative gas such as ozone.
In addition, in the case where the colorant is a pigment, the pigment is further required to be substantially insoluble in water or in an organic solvent, to have a good fastness to chemicals, and not to lose the preferred absorption characteristics it shows in a molecularly dispersed state even when used as pigment particles. Although the required properties described above can be controlled by adjusting the intensity of intramolelcular and intermolecular mutual action, both of them are in a trade-off relation with each other, thus being difficult to allow them to be compatible with each other.
Besides, in the case of using a pigment as the colorant, the pigment is additionally required to have a particle size and a particle shape necessary for realizing desired transparency, to have good fastness under the conditions of the environment wherein they are used, for example, fastness against light, heat, and an oxidative gas such as ozone, to have good fastness to an organic solvent and chemicals such as a sulfurous acid gas, and to be capable of being dispersed in a used medium to a level of fine particles, with the dispersed state being stable. In particular, there is a strong demand for a pigment which has a good yellow hue and a high tinctorial strength and is fast to light, heat, moisture, and active gases in the environment.
That is, in comparison with a dye which is required to have properties as colorant molecules, the pigment is required to have more properties, i.e., it is required to satisfy all of the above-mentioned requirements as a solid of an aggregate of a colorant (dispersion of fine particles) as well as the properties as molecules of a colorant molecule. As a result, a group of compounds which can be used as pigments are extremely limited in comparison with dyes. Even when high-performance dyes are converted to pigments, few of them can satisfy requirement for the properties as a dispersion of fine particles. Thus, such pigments are difficult to develop. This can be confirmed from the fact that the number of pigments registered in Color Index is no more than 1/10 of the number of dyes.
Azo pigments are excellent in hue and tinctorial strength which are characteristics of coloring, and hence they have widely been used in printing inks, inks for an inkjet system, and electrophotographic materials. Of the pigments, diarylide pigments are the most typically used yellow azo pigments. Examples of such diarylide pigments include C.I. pigment yellow 12, C.I. pigment yellow 13, and C.I. pigment yellow 17. However, the diarylide pigments are inferior in fastness, particularly light fastness, and hence they are inappropriate for prints which are to be stored for a long time.
In order to remove such defects, there have been disclosed azo pigments having a fastness improved by increasing molecular weight or by introducing a group having a strong intermolecular mutual action (see, for example, patent documents 1 to 3). However, even the improved pigments, for example, the pigments described in patent document 1 have the defect that they have still insufficient light fastness though improved to some extent, and pigments described in, for example, patent documents 2 and 3 have a greenish hue and a low tinctorial strength, thus being inferior in coloring characteristics.
Also, patent document 4 discloses colorants which have absorption characteristics of excellent color reproducibility and has a sufficient fastness. However, all of the specific compounds described in the patent document are soluble in water or in an organic solvent, thus being insufficient in resistance to chemicals.
Incidentally, patent document 5 describes an example of using a dye as a colorant and dissolving it in a water medium to use as a water-soluble ink for inkjet recording. Also, patent document 6 describes an anion-type monoazo compound characterized by light fastness. However, the level of image fastness of these is not satisfying at a high level, and they fail to provide a using manner as a pigment.
In the case of expressing a full-color image based on the subtractive color mixing process using three colors of yellow, magenta, and cyan or using four colors further including black, use of a pigment having an inferior fastness as a yellow pigment would change gray balance of the prints with the lapse of time, and use of a pigment having inferior coloring characteristics would reduce color reproducibility upon printing. Thus, in order to obtain prints which can maintain high color reproducibility for a long time, there have been desired a yellow pigment and a pigment dispersion which have both good coloring characteristics and good fastness.
Also, patent document 7 discloses colorants, as dyes, having absorption characteristics excellent in color reproducibility with an extremely high-level fastness.
On the other hand, with many of typical organic pigments, there exist polymorphic forms, and it has been known that such pigments take two or more crystal forms in spite of having the same chemical composition.
With some organic pigments, fine and size distribution-controlled particles can be obtained by selecting appropriate reaction conditions upon synthesis. There are pigments such as copper phthalocyanine green which are formed into pigments by allowing extremely fine and aggregated particles produced upon synthesis to grow in a subsequent step with size distribution being controlled, and pigments such as copper phthalocyanine blue pigment which are formed into pigments by pulverizing coarse and uneven particles produced upon synthesis in a subsequent step and controlling the size distribution. For example, a diketopyrrolopyrrole pigment is generally synthesized by reacting a succinic diester with an aromatic nitrile in an organic solvent (see, for example, patent document 8). The crude diketopyrrolopyrrole pigment is heat-treated in water or in an organic solvent, and then subjected to pulverization such as wet milling into a form appropriate for use (see, for example, patent document 9). For example, with a diketopyrrolopyrrole pigment of C.I. Pigment Red 254, an α-type crystal form and a β-type crystal form are known (see, for example, patent document 10). Also, with an azo pigment of C.I. Pigment Yellow 181, several crystal forms are known (see, for example, patent document 11).